JP4082292B discloses an engine for combusting a mixture gas inside a combustion chamber by self-ignition within a given operating range where an engine load and an engine speed are low. The engine combusts the mixture gas by spark-ignition within an operating range where the engine load is high and an operating range where the engine speed is higher than the given operating range.
Incidentally, combustion caused by compression ignition accompanies a relatively loud combustion noise. When the engine speed is high, NVH (Noise Vibration Harshness) of the engine exceeds an allowable value.